A plasma processing apparatus performs a processing such as a film formation on a substrate, e.g., a semiconductor wafer, which is mounted on a substrate table called a susceptor disposed in a process chamber, by using a plasma generated from a process gas introduced into the process chamber. A susceptor employed in the plasma processing apparatus includes a main body 1, an electrostatic chuck 2 disposed on the main body 1 and a focus ring 3 disposed along an outer periphery of the main body 1 in such a manner that it surrounds the electrostatic chucks 2, as shown in FIG. 8A for example. A wafer W is adsorptively fixed on the susceptor by the electrostatic chuck 2. The process chamber is maintained at a predetermined vacuum level and a predetermined high frequency power is applied from a high frequency power supply 4 connected to the main body 1 via a matching unit 4A. The plasma of the process gas generated between an upper electrode (not shown) and the susceptor is converged onto the wafer W by the focus ring 3. Further, formed within the main body 1 is a coolant passageway 1A through which a coolant circulates to cool the main body 1, which in turn cools the wafer W whose temperature rises during the plasma processing, so that the wafer W is maintained at a constant process temperature. Further prepared within the main body 1 is a gas channel 1B of a thermally conductive gas (e.g., He gas), which has openings at plural spots on a top surface of the main body 1. Formed in the electrostatic chuck 2 are through holes 2A corresponding to the gas channel 1B. The He gas is supplied between the electrostatic chuck 2 and the wafer W via the gas channel 1B and the through holes 2A to serve as a heat transfer medium in a narrow elongated gap existing between the electrostatic chuck 2 and the wafer W. As a result, a heat flow from the wafer W to the electrostatic chuck 2 and, further, to the main body 1 is facilitated, thereby efficiently cooling the wafer W. The electrostatic chuck 2 is of a plate shape obtained by sintering a ceramic such as alumina. Embedded in the electrostatic chuck 2 is an electrode plate 2B connected to a DC power supply 5. The electrostatic chuck 2 adsorbs the wafer W by an electrostatic force generated by a high voltage applied thereto from the DC power supply 5.
Since it is hard to produce a large sized thin plate by ceramic sintering, a manufacture of the electrostatic chuck 2 of a reasonable size for a large wafer W is also difficult. Therefore, recently, electrostatic chucks are fabricated by employing a ceramic thermal spray technique (see, for example, Japanese Patent No. 2971369). An electrostatic chuck obtained by ceramic thermal spray is hygroscopic due to pores existing between ceramic particles. Therefore, a pore sealing process is executed on the electrostatic chuck by using a silicone resin. The pore sealing process involves the steps of impregnating a silicone resin raw material solution, which is obtained by dissolving methyl sillyl triisocynate in ethyl acetate, into a thermally sprayed alumina layer of the electrostatic chuck; and then heating the electrostatic chuck in the atmosphere at a temperature of about 70° C. for about 8 hours, so that the methyl sillyl triisocynate is polymerized and cured to become the silicone resin. By repeating the impregnation step and the curing step plural times, the pore sealing process is completed.
However, in case of performing a plasma processing on the wafer W by applying a high frequency power at a high vacuum region (e.g., 100 mTorr) through the use of the ceramic sprayed electrostatic chuck on which the pore sealing process using the silicon resin is executed, there occurs a phenomenon that a surface temperature of the wafer is gradually reduced during plasma processing as the application time of the high frequency power increases, as illustrated by a graph {circle around (2)} in FIG. 7.